1Please note that the "What is RCU?" LWN series is an excellent place
2to start learning about RCU:
3 41. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ 52. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ 63. RCU part 3: the RCU API http://lwn.net/Articles/264090/ 74. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ 8 9 10What is RCU?
11 12RCU is a synchronization mechanism that was added to the Linux kernel
13during the 2.5 development effort that is optimized for read-mostly
14situations. Although RCU is actually quite simple once you understand it,
15getting there can sometimes be a challenge. Part of the problem is that
16most of the past descriptions of RCU have been written with the mistaken
17assumption that there is "one true way" to describe RCU. Instead,
18the experience has been that different people must take different paths
19to arrive at an understanding of RCU. This document provides several
20different paths, as follows:
21 221. RCU OVERVIEW
232. WHAT IS RCU'S CORE API?
243. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
254. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
265. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
276. ANALOGY WITH READER-WRITER LOCKING
287. FULL LIST OF RCU APIs
298. ANSWERS TO QUICK QUIZZES
30 31People who prefer starting with a conceptual overview should focus on
32Section 1, though most readers will profit by reading this section at
33some point. People who prefer to start with an API that they can then
34experiment with should focus on Section 2. People who prefer to start
35with example uses should focus on Sections 3 and 4. People who need to
36understand the RCU implementation should focus on Section 5, then dive
37into the kernel source code. People who reason best by analogy should
38focus on Section 6. Section 7 serves as an index to the docbook API
39documentation, and Section 8 is the traditional answer key.
40 41So, start with the section that makes the most sense to you and your
42preferred method of learning. If you need to know everything about
43everything, feel free to read the whole thing -- but if you are really
44that type of person, you have perused the source code and will therefore
45never need this document anyway. ;-)
46 47 481. RCU OVERVIEW
49 50The basic idea behind RCU is to split updates into "removal" and
51"reclamation" phases. The removal phase removes references to data items
52within a data structure (possibly by replacing them with references to
53new versions of these data items), and can run concurrently with readers.
54The reason that it is safe to run the removal phase concurrently with
55readers is the semantics of modern CPUs guarantee that readers will see
56either the old or the new version of the data structure rather than a
57partially updated reference. The reclamation phase does the work of reclaiming
58(e.g., freeing) the data items removed from the data structure during the
59removal phase. Because reclaiming data items can disrupt any readers
60concurrently referencing those data items, the reclamation phase must
61not start until readers no longer hold references to those data items.
62 63Splitting the update into removal and reclamation phases permits the
64updater to perform the removal phase immediately, and to defer the
65reclamation phase until all readers active during the removal phase have
66completed, either by blocking until they finish or by registering a
67callback that is invoked after they finish. Only readers that are active
68during the removal phase need be considered, because any reader starting
69after the removal phase will be unable to gain a reference to the removed
70data items, and therefore cannot be disrupted by the reclamation phase.
71 72So the typical RCU update sequence goes something like the following:
73 74a. Remove pointers to a data structure, so that subsequent
75 readers cannot gain a reference to it.
76 77b. Wait for all previous readers to complete their RCU read-side
78 critical sections.
79 80c. At this point, there cannot be any readers who hold references
81 to the data structure, so it now may safely be reclaimed
82 (e.g., kfree()d).
83 84Step (b) above is the key idea underlying RCU's deferred destruction.
85The ability to wait until all readers are done allows RCU readers to
86use much lighter-weight synchronization, in some cases, absolutely no
87synchronization at all. In contrast, in more conventional lock-based
88schemes, readers must use heavy-weight synchronization in order to
89prevent an updater from deleting the data structure out from under them.
90This is because lock-based updaters typically update data items in place,
91and must therefore exclude readers. In contrast, RCU-based updaters
92typically take advantage of the fact that writes to single aligned
93pointers are atomic on modern CPUs, allowing atomic insertion, removal,
94and replacement of data items in a linked structure without disrupting
95readers. Concurrent RCU readers can then continue accessing the old
96versions, and can dispense with the atomic operations, memory barriers,
97and communications cache misses that are so expensive on present-day
98SMP computer systems, even in absence of lock contention.
99 100In the three-step procedure shown above, the updater is performing both
101the removal and the reclamation step, but it is often helpful for an
102entirely different thread to do the reclamation, as is in fact the case
103in the Linux kernel's directory-entry cache (dcache). Even if the same
104thread performs both the update step (step (a) above) and the reclamation
105step (step (c) above), it is often helpful to think of them separately.
106For example, RCU readers and updaters need not communicate at all,
107but RCU provides implicit low-overhead communication between readers
108and reclaimers, namely, in step (b) above.
109 110So how the heck can a reclaimer tell when a reader is done, given
111that readers are not doing any sort of synchronization operations???
112Read on to learn about how RCU's API makes this easy.
113 114 1152. WHAT IS RCU'S CORE API?
116 117The core RCU API is quite small:
118 119a. rcu_read_lock()
120b. rcu_read_unlock()
121c. synchronize_rcu() / call_rcu()
122d. rcu_assign_pointer()
123e. rcu_dereference()
124 125There are many other members of the RCU API, but the rest can be
126expressed in terms of these five, though most implementations instead
127express synchronize_rcu() in terms of the call_rcu() callback API.
128 129The five core RCU APIs are described below, the other 18 will be enumerated
130later. See the kernel docbook documentation for more info, or look directly
131at the function header comments.
132 133rcu_read_lock()
134 135 void rcu_read_lock(void);
136 137 Used by a reader to inform the reclaimer that the reader is
138 entering an RCU read-side critical section. It is illegal
139 to block while in an RCU read-side critical section, though
140 kernels built with CONFIG_TREE_PREEMPT_RCU can preempt RCU
141 read-side critical sections. Any RCU-protected data structure
142 accessed during an RCU read-side critical section is guaranteed to
143 remain unreclaimed for the full duration of that critical section.
144 Reference counts may be used in conjunction with RCU to maintain
145 longer-term references to data structures.
146 147rcu_read_unlock()
148 149 void rcu_read_unlock(void);
150 151 Used by a reader to inform the reclaimer that the reader is
152 exiting an RCU read-side critical section. Note that RCU
153 read-side critical sections may be nested and/or overlapping.
154 155synchronize_rcu()
156 157 void synchronize_rcu(void);
158 159 Marks the end of updater code and the beginning of reclaimer
160 code. It does this by blocking until all pre-existing RCU
161 read-side critical sections on all CPUs have completed.
162 Note that synchronize_rcu() will -not- necessarily wait for
163 any subsequent RCU read-side critical sections to complete.
164 For example, consider the following sequence of events:
165 166 CPU 0 CPU 1 CPU 2
167 ----------------- ------------------------- ---------------
168 1. rcu_read_lock()
169 2. enters synchronize_rcu()
170 3. rcu_read_lock()
171 4. rcu_read_unlock()
172 5. exits synchronize_rcu()
173 6. rcu_read_unlock()
174 175 To reiterate, synchronize_rcu() waits only for ongoing RCU
176 read-side critical sections to complete, not necessarily for
177 any that begin after synchronize_rcu() is invoked.
178 179 Of course, synchronize_rcu() does not necessarily return
180 -immediately- after the last pre-existing RCU read-side critical
181 section completes. For one thing, there might well be scheduling
182 delays. For another thing, many RCU implementations process
183 requests in batches in order to improve efficiencies, which can
184 further delay synchronize_rcu().
185 186 Since synchronize_rcu() is the API that must figure out when
187 readers are done, its implementation is key to RCU. For RCU
188 to be useful in all but the most read-intensive situations,
189 synchronize_rcu()'s overhead must also be quite small.
190 191 The call_rcu() API is a callback form of synchronize_rcu(),
192 and is described in more detail in a later section. Instead of
193 blocking, it registers a function and argument which are invoked
194 after all ongoing RCU read-side critical sections have completed.
195 This callback variant is particularly useful in situations where
196 it is illegal to block or where update-side performance is
197 critically important.
198 199 However, the call_rcu() API should not be used lightly, as use
200 of the synchronize_rcu() API generally results in simpler code.
201 In addition, the synchronize_rcu() API has the nice property
202 of automatically limiting update rate should grace periods
203 be delayed. This property results in system resilience in face
204 of denial-of-service attacks. Code using call_rcu() should limit
205 update rate in order to gain this same sort of resilience. See
206 checklist.txt for some approaches to limiting the update rate.
207 208rcu_assign_pointer()
209 210 typeof(p) rcu_assign_pointer(p, typeof(p) v);
211 212 Yes, rcu_assign_pointer() -is- implemented as a macro, though it
213 would be cool to be able to declare a function in this manner.
214 (Compiler experts will no doubt disagree.)
215 216 The updater uses this function to assign a new value to an
217 RCU-protected pointer, in order to safely communicate the change
218 in value from the updater to the reader. This function returns
219 the new value, and also executes any memory-barrier instructions
220 required for a given CPU architecture.
221 222 Perhaps just as important, it serves to document (1) which
223 pointers are protected by RCU and (2) the point at which a
224 given structure becomes accessible to other CPUs. That said,
225 rcu_assign_pointer() is most frequently used indirectly, via
226 the _rcu list-manipulation primitives such as list_add_rcu().
227 228rcu_dereference()
229 230 typeof(p) rcu_dereference(p);
231 232 Like rcu_assign_pointer(), rcu_dereference() must be implemented
233 as a macro.
234 235 The reader uses rcu_dereference() to fetch an RCU-protected
236 pointer, which returns a value that may then be safely
237 dereferenced. Note that rcu_deference() does not actually
238 dereference the pointer, instead, it protects the pointer for
239 later dereferencing. It also executes any needed memory-barrier
240 instructions for a given CPU architecture. Currently, only Alpha
241 needs memory barriers within rcu_dereference() -- on other CPUs,
242 it compiles to nothing, not even a compiler directive.
243 244 Common coding practice uses rcu_dereference() to copy an
245 RCU-protected pointer to a local variable, then dereferences
246 this local variable, for example as follows:
247 248 p = rcu_dereference(head.next);
249 return p->data;
250 251 However, in this case, one could just as easily combine these
252 into one statement:
253 254 return rcu_dereference(head.next)->data;
255 256 If you are going to be fetching multiple fields from the
257 RCU-protected structure, using the local variable is of
258 course preferred. Repeated rcu_dereference() calls look
259 ugly and incur unnecessary overhead on Alpha CPUs.
260 261 Note that the value returned by rcu_dereference() is valid
262 only within the enclosing RCU read-side critical section.
263 For example, the following is -not- legal:
264 265 rcu_read_lock();
266 p = rcu_dereference(head.next);
267 rcu_read_unlock();
268 x = p->address;
269 rcu_read_lock();
270 y = p->data;
271 rcu_read_unlock();
272 273 Holding a reference from one RCU read-side critical section
274 to another is just as illegal as holding a reference from
275 one lock-based critical section to another! Similarly,
276 using a reference outside of the critical section in which
277 it was acquired is just as illegal as doing so with normal
278 locking.
279 280 As with rcu_assign_pointer(), an important function of
281 rcu_dereference() is to document which pointers are protected by
282 RCU, in particular, flagging a pointer that is subject to changing
283 at any time, including immediately after the rcu_dereference().
284 And, again like rcu_assign_pointer(), rcu_dereference() is
285 typically used indirectly, via the _rcu list-manipulation
286 primitives, such as list_for_each_entry_rcu().
287 288The following diagram shows how each API communicates among the
289reader, updater, and reclaimer.
290 291 292 rcu_assign_pointer()
293 +--------+
294 +---------------------->| reader |---------+
295 | +--------+ |
296 | | |
297 | | | Protect:
298 | | | rcu_read_lock()
299 | | | rcu_read_unlock()
300 | rcu_dereference() | |
301 +---------+ | |
302 | updater |<---------------------+ |
303 +---------+ V
304 | +-----------+
305 +----------------------------------->| reclaimer |
306 +-----------+
307 Defer:
308 synchronize_rcu() & call_rcu()
309 310 311The RCU infrastructure observes the time sequence of rcu_read_lock(),
312rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
313order to determine when (1) synchronize_rcu() invocations may return
314to their callers and (2) call_rcu() callbacks may be invoked. Efficient
315implementations of the RCU infrastructure make heavy use of batching in
316order to amortize their overhead over many uses of the corresponding APIs.
317 318There are no fewer than three RCU mechanisms in the Linux kernel; the
319diagram above shows the first one, which is by far the most commonly used.
320The rcu_dereference() and rcu_assign_pointer() primitives are used for
321all three mechanisms, but different defer and protect primitives are
322used as follows:
323 324 Defer Protect
325 326a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
327 call_rcu() rcu_dereference()
328 329b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
330 rcu_dereference_bh()
331 332c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
333 preempt_disable() / preempt_enable()
334 local_irq_save() / local_irq_restore()
335 hardirq enter / hardirq exit
336 NMI enter / NMI exit
337 rcu_dereference_sched()
338 339These three mechanisms are used as follows:
340 341a. RCU applied to normal data structures.
342 343b. RCU applied to networking data structures that may be subjected
344 to remote denial-of-service attacks.
345 346c. RCU applied to scheduler and interrupt/NMI-handler tasks.
347 348Again, most uses will be of (a). The (b) and (c) cases are important
349for specialized uses, but are relatively uncommon.
350 351 3523. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
353 354This section shows a simple use of the core RCU API to protect a
355global pointer to a dynamically allocated structure. More-typical
356uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
357 358 struct foo {
359 int a;
360 char b;
361 long c;
362 };
363 DEFINE_SPINLOCK(foo_mutex);
364 365 struct foo *gbl_foo;
366 367 /*
368 * Create a new struct foo that is the same as the one currently
369 * pointed to by gbl_foo, except that field "a" is replaced
370 * with "new_a". Points gbl_foo to the new structure, and
371 * frees up the old structure after a grace period.
372 *
373 * Uses rcu_assign_pointer() to ensure that concurrent readers
374 * see the initialized version of the new structure.
375 *
376 * Uses synchronize_rcu() to ensure that any readers that might
377 * have references to the old structure complete before freeing
378 * the old structure.
379 */
380 void foo_update_a(int new_a)
381 {
382 struct foo *new_fp;
383 struct foo *old_fp;
384 385 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
386 spin_lock(&foo_mutex);
387 old_fp = gbl_foo;
388 *new_fp = *old_fp;
389 new_fp->a = new_a;
390 rcu_assign_pointer(gbl_foo, new_fp);
391 spin_unlock(&foo_mutex);
392 synchronize_rcu();
393 kfree(old_fp);
394 }
395 396 /*
397 * Return the value of field "a" of the current gbl_foo
398 * structure. Use rcu_read_lock() and rcu_read_unlock()
399 * to ensure that the structure does not get deleted out
400 * from under us, and use rcu_dereference() to ensure that
401 * we see the initialized version of the structure (important
402 * for DEC Alpha and for people reading the code).
403 */
404 int foo_get_a(void)
405 {
406 int retval;
407 408 rcu_read_lock();
409 retval = rcu_dereference(gbl_foo)->a;
410 rcu_read_unlock();
411 return retval;
412 }
413 414So, to sum up:
415 416o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
417 read-side critical sections.
418 419o Within an RCU read-side critical section, use rcu_dereference()
420 to dereference RCU-protected pointers.
421 422o Use some solid scheme (such as locks or semaphores) to
423 keep concurrent updates from interfering with each other.
424 425o Use rcu_assign_pointer() to update an RCU-protected pointer.
426 This primitive protects concurrent readers from the updater,
427 -not- concurrent updates from each other! You therefore still
428 need to use locking (or something similar) to keep concurrent
429 rcu_assign_pointer() primitives from interfering with each other.
430 431o Use synchronize_rcu() -after- removing a data element from an
432 RCU-protected data structure, but -before- reclaiming/freeing
433 the data element, in order to wait for the completion of all
434 RCU read-side critical sections that might be referencing that
435 data item.
436 437See checklist.txt for additional rules to follow when using RCU.
438And again, more-typical uses of RCU may be found in listRCU.txt,
439arrayRCU.txt, and NMI-RCU.txt.
440 441 4424. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
443 444In the example above, foo_update_a() blocks until a grace period elapses.
445This is quite simple, but in some cases one cannot afford to wait so
446long -- there might be other high-priority work to be done.
447 448In such cases, one uses call_rcu() rather than synchronize_rcu().
449The call_rcu() API is as follows:
450 451 void call_rcu(struct rcu_head * head,
452 void (*func)(struct rcu_head *head));
453 454This function invokes func(head) after a grace period has elapsed.
455This invocation might happen from either softirq or process context,
456so the function is not permitted to block. The foo struct needs to
457have an rcu_head structure added, perhaps as follows:
458 459 struct foo {
460 int a;
461 char b;
462 long c;
463 struct rcu_head rcu;
464 };
465 466The foo_update_a() function might then be written as follows:
467 468 /*
469 * Create a new struct foo that is the same as the one currently
470 * pointed to by gbl_foo, except that field "a" is replaced
471 * with "new_a". Points gbl_foo to the new structure, and
472 * frees up the old structure after a grace period.
473 *
474 * Uses rcu_assign_pointer() to ensure that concurrent readers
475 * see the initialized version of the new structure.
476 *
477 * Uses call_rcu() to ensure that any readers that might have
478 * references to the old structure complete before freeing the
479 * old structure.
480 */
481 void foo_update_a(int new_a)
482 {
483 struct foo *new_fp;
484 struct foo *old_fp;
485 486 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
487 spin_lock(&foo_mutex);
488 old_fp = gbl_foo;
489 *new_fp = *old_fp;
490 new_fp->a = new_a;
491 rcu_assign_pointer(gbl_foo, new_fp);
492 spin_unlock(&foo_mutex);
493 call_rcu(&old_fp->rcu, foo_reclaim);
494 }
495 496The foo_reclaim() function might appear as follows:
497 498 void foo_reclaim(struct rcu_head *rp)
499 {
500 struct foo *fp = container_of(rp, struct foo, rcu);
501 502 kfree(fp);
503 }
504 505The container_of() primitive is a macro that, given a pointer into a
506struct, the type of the struct, and the pointed-to field within the
507struct, returns a pointer to the beginning of the struct.
508 509The use of call_rcu() permits the caller of foo_update_a() to
510immediately regain control, without needing to worry further about the
511old version of the newly updated element. It also clearly shows the
512RCU distinction between updater, namely foo_update_a(), and reclaimer,
513namely foo_reclaim().
514 515The summary of advice is the same as for the previous section, except
516that we are now using call_rcu() rather than synchronize_rcu():
517 518o Use call_rcu() -after- removing a data element from an
519 RCU-protected data structure in order to register a callback
520 function that will be invoked after the completion of all RCU
521 read-side critical sections that might be referencing that
522 data item.
523 524Again, see checklist.txt for additional rules governing the use of RCU.
525 526 5275. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
528 529One of the nice things about RCU is that it has extremely simple "toy"
530implementations that are a good first step towards understanding the
531production-quality implementations in the Linux kernel. This section
532presents two such "toy" implementations of RCU, one that is implemented
533in terms of familiar locking primitives, and another that more closely
534resembles "classic" RCU. Both are way too simple for real-world use,
535lacking both functionality and performance. However, they are useful
536in getting a feel for how RCU works. See kernel/rcupdate.c for a
537production-quality implementation, and see:
538 539http://www.rdrop.com/users/paulmck/RCU 540 541for papers describing the Linux kernel RCU implementation. The OLS'01
542and OLS'02 papers are a good introduction, and the dissertation provides
543more details on the current implementation as of early 2004.
544 545 5465A. "TOY" IMPLEMENTATION #1: LOCKING
547 548This section presents a "toy" RCU implementation that is based on
549familiar locking primitives. Its overhead makes it a non-starter for
550real-life use, as does its lack of scalability. It is also unsuitable
551for realtime use, since it allows scheduling latency to "bleed" from
552one read-side critical section to another.
553 554However, it is probably the easiest implementation to relate to, so is
555a good starting point.
556 557It is extremely simple:
558 559 static DEFINE_RWLOCK(rcu_gp_mutex);
560 561 void rcu_read_lock(void)
562 {
563 read_lock(&rcu_gp_mutex);
564 }
565 566 void rcu_read_unlock(void)
567 {
568 read_unlock(&rcu_gp_mutex);
569 }
570 571 void synchronize_rcu(void)
572 {
573 write_lock(&rcu_gp_mutex);
574 write_unlock(&rcu_gp_mutex);
575 }
576 577[You can ignore rcu_assign_pointer() and rcu_dereference() without
578missing much. But here they are anyway. And whatever you do, don't
579forget about them when submitting patches making use of RCU!]
580 581 #define rcu_assign_pointer(p, v) ({ \
582 smp_wmb(); \
583 (p) = (v); \
584 })
585 586 #define rcu_dereference(p) ({ \
587 typeof(p) _________p1 = p; \
588 smp_read_barrier_depends(); \
589 (_________p1); \
590 })
591 592 593The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
594and release a global reader-writer lock. The synchronize_rcu()
595primitive write-acquires this same lock, then immediately releases
596it. This means that once synchronize_rcu() exits, all RCU read-side
597critical sections that were in progress before synchronize_rcu() was
598called are guaranteed to have completed -- there is no way that
599synchronize_rcu() would have been able to write-acquire the lock
600otherwise.
601 602It is possible to nest rcu_read_lock(), since reader-writer locks may
603be recursively acquired. Note also that rcu_read_lock() is immune
604from deadlock (an important property of RCU). The reason for this is
605that the only thing that can block rcu_read_lock() is a synchronize_rcu().
606But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
607so there can be no deadlock cycle.
608 609Quick Quiz #1: Why is this argument naive? How could a deadlock
610 occur when using this algorithm in a real-world Linux
611 kernel? How could this deadlock be avoided?
612 613 6145B. "TOY" EXAMPLE #2: CLASSIC RCU
615 616This section presents a "toy" RCU implementation that is based on
617"classic RCU". It is also short on performance (but only for updates) and
618on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
619kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
620are the same as those shown in the preceding section, so they are omitted.
621 622 void rcu_read_lock(void) { }
623 624 void rcu_read_unlock(void) { }
625 626 void synchronize_rcu(void)
627 {
628 int cpu;
629 630 for_each_possible_cpu(cpu)
631 run_on(cpu);
632 }
633 634Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
635This is the great strength of classic RCU in a non-preemptive kernel:
636read-side overhead is precisely zero, at least on non-Alpha CPUs.
637And there is absolutely no way that rcu_read_lock() can possibly
638participate in a deadlock cycle!
639 640The implementation of synchronize_rcu() simply schedules itself on each
641CPU in turn. The run_on() primitive can be implemented straightforwardly
642in terms of the sched_setaffinity() primitive. Of course, a somewhat less
643"toy" implementation would restore the affinity upon completion rather
644than just leaving all tasks running on the last CPU, but when I said
645"toy", I meant -toy-!
646 647So how the heck is this supposed to work???
648 649Remember that it is illegal to block while in an RCU read-side critical
650section. Therefore, if a given CPU executes a context switch, we know
651that it must have completed all preceding RCU read-side critical sections.
652Once -all- CPUs have executed a context switch, then -all- preceding
653RCU read-side critical sections will have completed.
654 655So, suppose that we remove a data item from its structure and then invoke
656synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
657that there are no RCU read-side critical sections holding a reference
658to that data item, so we can safely reclaim it.
659 660Quick Quiz #2: Give an example where Classic RCU's read-side
661 overhead is -negative-.
662 663Quick Quiz #3: If it is illegal to block in an RCU read-side
664 critical section, what the heck do you do in
665 PREEMPT_RT, where normal spinlocks can block???
666 667 6686. ANALOGY WITH READER-WRITER LOCKING
669 670Although RCU can be used in many different ways, a very common use of
671RCU is analogous to reader-writer locking. The following unified
672diff shows how closely related RCU and reader-writer locking can be.
673 674 @@ -13,15 +14,15 @@
675 struct list_head *lp;
676 struct el *p;
677 678 - read_lock();
679 - list_for_each_entry(p, head, lp) {
680 + rcu_read_lock();
681 + list_for_each_entry_rcu(p, head, lp) {
682 if (p->key == key) {
683 *result = p->data;
684 - read_unlock();
685 + rcu_read_unlock();
686 return 1;
687 }
688 }
689 - read_unlock();
690 + rcu_read_unlock();
691 return 0;
692 }
693 694 @@ -29,15 +30,16 @@
695 {
696 struct el *p;
697 698 - write_lock(&listmutex);
699 + spin_lock(&listmutex);
700 list_for_each_entry(p, head, lp) {
701 if (p->key == key) {
702 - list_del(&p->list);
703 - write_unlock(&listmutex);
704 + list_del_rcu(&p->list);
705 + spin_unlock(&listmutex);
706 + synchronize_rcu();
707 kfree(p);
708 return 1;
709 }
710 }
711 - write_unlock(&listmutex);
712 + spin_unlock(&listmutex);
713 return 0;
714 }
715 716Or, for those who prefer a side-by-side listing:
717 718 1 struct el { 1 struct el {
719 2 struct list_head list; 2 struct list_head list;
720 3 long key; 3 long key;
721 4 spinlock_t mutex; 4 spinlock_t mutex;
722 5 int data; 5 int data;
723 6 /* Other data fields */ 6 /* Other data fields */
724 7 }; 7 };
725 8 spinlock_t listmutex; 8 spinlock_t listmutex;
726 9 struct el head; 9 struct el head;
727 728 1 int search(long key, int *result) 1 int search(long key, int *result)
729 2 { 2 {
730 3 struct list_head *lp; 3 struct list_head *lp;
731 4 struct el *p; 4 struct el *p;
732 5 5
733 6 read_lock(); 6 rcu_read_lock();
734 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
735 8 if (p->key == key) { 8 if (p->key == key) {
736 9 *result = p->data; 9 *result = p->data;
73710 read_unlock(); 10 rcu_read_unlock();
73811 return 1; 11 return 1;
73912 } 12 }
74013 } 13 }
74114 read_unlock(); 14 rcu_read_unlock();
74215 return 0; 15 return 0;
74316 } 16 }
744 745 1 int delete(long key) 1 int delete(long key)
746 2 { 2 {
747 3 struct el *p; 3 struct el *p;
748 4 4
749 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
750 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
751 7 if (p->key == key) { 7 if (p->key == key) {
752 8 list_del(&p->list); 8 list_del_rcu(&p->list);
753 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
754 10 synchronize_rcu();
75510 kfree(p); 11 kfree(p);
75611 return 1; 12 return 1;
75712 } 13 }
75813 } 14 }
75914 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
76015 return 0; 16 return 0;
76116 } 17 }
762 763Either way, the differences are quite small. Read-side locking moves
764to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
765a reader-writer lock to a simple spinlock, and a synchronize_rcu()
766precedes the kfree().
767 768However, there is one potential catch: the read-side and update-side
769critical sections can now run concurrently. In many cases, this will
770not be a problem, but it is necessary to check carefully regardless.
771For example, if multiple independent list updates must be seen as
772a single atomic update, converting to RCU will require special care.
773 774Also, the presence of synchronize_rcu() means that the RCU version of
775delete() can now block. If this is a problem, there is a callback-based
776mechanism that never blocks, namely call_rcu(), that can be used in
777place of synchronize_rcu().
778 779 7807. FULL LIST OF RCU APIs
781 782The RCU APIs are documented in docbook-format header comments in the
783Linux-kernel source code, but it helps to have a full list of the
784APIs, since there does not appear to be a way to categorize them
785in docbook. Here is the list, by category.
786 787RCU list traversal:
788 789 list_for_each_entry_rcu
790 hlist_for_each_entry_rcu
791 hlist_nulls_for_each_entry_rcu
792 793 list_for_each_continue_rcu (to be deprecated in favor of new
794 list_for_each_entry_continue_rcu)
795 796RCU pointer/list update:
797 798 rcu_assign_pointer
799 list_add_rcu
800 list_add_tail_rcu
801 list_del_rcu
802 list_replace_rcu
803 hlist_del_rcu
804 hlist_add_after_rcu
805 hlist_add_before_rcu
806 hlist_add_head_rcu
807 hlist_replace_rcu
808 list_splice_init_rcu()
809 810RCU: Critical sections Grace period Barrier
811 812 rcu_read_lock synchronize_net rcu_barrier
813 rcu_read_unlock synchronize_rcu
814 rcu_dereference synchronize_rcu_expedited
815 call_rcu
816 817 818bh: Critical sections Grace period Barrier
819 820 rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
821 rcu_read_unlock_bh synchronize_rcu_bh
822 rcu_dereference_bh synchronize_rcu_bh_expedited
823 824 825sched: Critical sections Grace period Barrier
826 827 rcu_read_lock_sched synchronize_sched rcu_barrier_sched
828 rcu_read_unlock_sched call_rcu_sched
829 [preempt_disable] synchronize_sched_expedited
830 [and friends]
831 rcu_dereference_sched
832 833 834SRCU: Critical sections Grace period Barrier
835 836 srcu_read_lock synchronize_srcu srcu_barrier
837 srcu_read_unlock call_srcu
838 srcu_read_lock_raw synchronize_srcu_expedited
839 srcu_read_unlock_raw
840 srcu_dereference
841 842SRCU: Initialization/cleanup
843 init_srcu_struct
844 cleanup_srcu_struct
845 846All: lockdep-checked RCU-protected pointer access
847 848 rcu_dereference_check
849 rcu_dereference_protected
850 rcu_access_pointer
851 852See the comment headers in the source code (or the docbook generated
853from them) for more information.
854 855However, given that there are no fewer than four families of RCU APIs
856in the Linux kernel, how do you choose which one to use? The following
857list can be helpful:
858 859a. Will readers need to block? If so, you need SRCU.
860 861b. Is it necessary to start a read-side critical section in a
862 hardirq handler or exception handler, and then to complete
863 this read-side critical section in the task that was
864 interrupted? If so, you need SRCU's srcu_read_lock_raw() and
865 srcu_read_unlock_raw() primitives.
866 867c. What about the -rt patchset? If readers would need to block
868 in an non-rt kernel, you need SRCU. If readers would block
869 in a -rt kernel, but not in a non-rt kernel, SRCU is not
870 necessary.
871 872d. Do you need to treat NMI handlers, hardirq handlers,
873 and code segments with preemption disabled (whether
874 via preempt_disable(), local_irq_save(), local_bh_disable(),
875 or some other mechanism) as if they were explicit RCU readers?
876 If so, RCU-sched is the only choice that will work for you.
877 878e. Do you need RCU grace periods to complete even in the face
879 of softirq monopolization of one or more of the CPUs? For
880 example, is your code subject to network-based denial-of-service
881 attacks? If so, you need RCU-bh.
882 883f. Is your workload too update-intensive for normal use of
884 RCU, but inappropriate for other synchronization mechanisms?
885 If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
886 887g. Do you need read-side critical sections that are respected
888 even though they are in the middle of the idle loop, during
889 user-mode execution, or on an offlined CPU? If so, SRCU is the
890 only choice that will work for you.
891 892h. Otherwise, use RCU.
893 894Of course, this all assumes that you have determined that RCU is in fact
895the right tool for your job.
896 897 8988. ANSWERS TO QUICK QUIZZES
899 900Quick Quiz #1: Why is this argument naive? How could a deadlock
901 occur when using this algorithm in a real-world Linux
902 kernel? [Referring to the lock-based "toy" RCU
903 algorithm.]
904 905Answer: Consider the following sequence of events:
906 907 1. CPU 0 acquires some unrelated lock, call it
908 "problematic_lock", disabling irq via
909 spin_lock_irqsave().
910 911 2. CPU 1 enters synchronize_rcu(), write-acquiring
912 rcu_gp_mutex.
913 914 3. CPU 0 enters rcu_read_lock(), but must wait
915 because CPU 1 holds rcu_gp_mutex.
916 917 4. CPU 1 is interrupted, and the irq handler
918 attempts to acquire problematic_lock.
919 920 The system is now deadlocked.
921 922 One way to avoid this deadlock is to use an approach like
923 that of CONFIG_PREEMPT_RT, where all normal spinlocks
924 become blocking locks, and all irq handlers execute in
925 the context of special tasks. In this case, in step 4
926 above, the irq handler would block, allowing CPU 1 to
927 release rcu_gp_mutex, avoiding the deadlock.
928 929 Even in the absence of deadlock, this RCU implementation
930 allows latency to "bleed" from readers to other
931 readers through synchronize_rcu(). To see this,
932 consider task A in an RCU read-side critical section
933 (thus read-holding rcu_gp_mutex), task B blocked
934 attempting to write-acquire rcu_gp_mutex, and
935 task C blocked in rcu_read_lock() attempting to
936 read_acquire rcu_gp_mutex. Task A's RCU read-side
937 latency is holding up task C, albeit indirectly via
938 task B.
939 940 Realtime RCU implementations therefore use a counter-based
941 approach where tasks in RCU read-side critical sections
942 cannot be blocked by tasks executing synchronize_rcu().
943 944Quick Quiz #2: Give an example where Classic RCU's read-side
945 overhead is -negative-.
946 947Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
948 kernel where a routing table is used by process-context
949 code, but can be updated by irq-context code (for example,
950 by an "ICMP REDIRECT" packet). The usual way of handling
951 this would be to have the process-context code disable
952 interrupts while searching the routing table. Use of
953 RCU allows such interrupt-disabling to be dispensed with.
954 Thus, without RCU, you pay the cost of disabling interrupts,
955 and with RCU you don't.
956 957 One can argue that the overhead of RCU in this
958 case is negative with respect to the single-CPU
959 interrupt-disabling approach. Others might argue that
960 the overhead of RCU is merely zero, and that replacing
961 the positive overhead of the interrupt-disabling scheme
962 with the zero-overhead RCU scheme does not constitute
963 negative overhead.
964 965 In real life, of course, things are more complex. But
966 even the theoretical possibility of negative overhead for
967 a synchronization primitive is a bit unexpected. ;-)
968 969Quick Quiz #3: If it is illegal to block in an RCU read-side
970 critical section, what the heck do you do in
971 PREEMPT_RT, where normal spinlocks can block???
972 973Answer: Just as PREEMPT_RT permits preemption of spinlock
974 critical sections, it permits preemption of RCU
975 read-side critical sections. It also permits
976 spinlocks blocking while in RCU read-side critical
977 sections.
978 979 Why the apparent inconsistency? Because it is it
980 possible to use priority boosting to keep the RCU
981 grace periods short if need be (for example, if running
982 short of memory). In contrast, if blocking waiting
983 for (say) network reception, there is no way to know
984 what should be boosted. Especially given that the
985 process we need to boost might well be a human being
986 who just went out for a pizza or something. And although
987 a computer-operated cattle prod might arouse serious
988 interest, it might also provoke serious objections.
989 Besides, how does the computer know what pizza parlor
990 the human being went to???
991 992 993ACKNOWLEDGEMENTS
994 995My thanks to the people who helped make this human-readable, including
996Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
997 998 999For more information, see http://www.rdrop.com/users/paulmck/RCU.
1000